Winter has already launched its frontal assault and once again the roads prove a dangerous challenge. While the best thing to do when the roads freeze is to stay home and keep warm, you may find yourself forced to drive. If that's the case, it helps to know how the ice affects your vehicle.

Please keep in mind that intentionally driving in icy conditions is dangerous no matter where you live. While many cities are prepared for inclement weather, some are not. If you're not sure you can drive in winter conditions, stay home.

De-Ice Your Windshields

One of the biggest dangers to driving in cold weather is visual impairment. There are plenty of ways to prevent ice and frost from building up on your car, but if you don't happen to deal with the problem ahead of time, (some of us are bad at checking weather apps, okay?), there are some good and very, very bad ways to clear ice from your windshield.

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Turn up the heat: If you're starting to head out on your journey and your car is iced over, turn the heat and defrosters up and wait. While being patient is hard, you won't just remove the initial ice by heating up your vehicle, you'll make it harder for more to accumulate while you drive.

Apply salt: Salt lowers the freezing point of water, which is why it's so useful for keeping streets clear. The same can work on your windshield if you're in a super hurry. Note, we're talking about large, grainy salt. Not the type you put on a roast chicken.

Broadly speaking, though, you're not going to be able to speed up the process too much. There are some tricks, however, that you should never try. For starters: do not pour boiling water on your windows. Pouring hot water on a cold windshield can potentially shatter the glass. Stack Exchange user Iserni explains the science behind why this happens:

When we pour boiling water on the glass, the outer part rapidly heats to around 80-100 °C (let's say 80), while the inner part may remain at -20 °C, thanks to the inner PVB sandwiched layer, for a comparatively long time (tenths of a second).

During this time, if we consider an area maybe thirty centimeters across, the heated zone undergoes a thermal expansion to the tune of 100 K (-20 to 80) by 9 millionths per K per unit of length.

Nine millionths may seem paltry, but multiplied by 100 K and further multiplied by 300 millimeters become 270,000 millionths of a millimeter - that is, one quarter of a millimeter.

Given the modulus of elasticity of seventy thousand megaPascal, this deformation should equal a "pressure" of around six hundred atmospheres: i.e., the same quarter-millimeter-per-thirty-centimeters deformation can be obtained by applying pressures of the same order of those eighteen thousand feet underwater.

It is not too surprising that in such a condition the PVB layer would come unraveled, and/or the outer layer of glass shattered, especially if there's already a flaw or chip to provide a weak point.

For those that might have glazed over, the short version is that applying excessive heat to a cold surface causes rapid expansion of the molecules on the surface, but doesn't immediately heat the inner parts of the glass. If there's even a small, invisible fracture in the windshield (or if the temperature change is too drastic), it can rip cracks in your windshield as the inner layers of glass struggle to heat up as quickly as the surface you poured water on.

For this reason, merely warm or room-temperature water can be less risky than boiling water. However, the new problem you face is that if the warm water doesn't melt the ice and disperse fast enough, and if the outside temperature is cold enough, you could end up creating an even thicker sheet of ice that's harder to remove. In general, pouring any temperature water on your windshield—while potentially anecdotally effective under the right circumstances—is far too risky and not worth the extra few minutes you might have saved.

Maintain Control and Avoid Skidding on Ice

Under normal conditions, you car drives along nice and safely on a fine balance of carefully negotiated forces between itself and the road. When you accelerate, your car's tires push on the road and the road pushes back, resulting in forward motion. As you're moving, you can apply sideways force by turning your wheels to steer your car in the direction of the turn. When you need to stop, your brakes apply force to your tires, increasing friction with the road and bringing your vehicle to a stop. When the roads are icy or slick, none of this works very well.

Keep Track of Your Traction: If you find yourself on icy roads, start by taking it slow. The lack of friction on the road increases your stopping time. We've shared this tip before, but the physics behind it shows just how important this is: Under normal circumstances, increased speed dramatically raises the length of road you need to come to a full stop. As the Physics Classroom shows, merely jumping from about 22mph to 45mph can increase your stopping distance by four fold (5 meters to 20 meters, by their calculations). When ice forms on the road, this can take even longer if you lose traction. For this reason, extend your following distance when you're behind other cars on icy roads.

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If you have preparation time, you should also consider either getting snow tires or chains. Chains can work better in the short term on icy roads for providing traction, but they require you to put them on and take them off as needed. Alternatively, you can find winter tires that will naturally boost the traction of your tires. They also are made of a softer rubber that handles better in temperatures below 45 degrees. They're more expensive, but if you live in an area with frequent cold spells, they may be worth looking into. Both snow tires and chains are solid options with pros and cons to each, depending on your circumstances.

Manage Your Momentum: Under normal circumstances, your car will start and stop when you tell it to because it's designed with friction in mind. When you're driving on ice, you're inside a several ton hunk of metal that's essentially gliding across ice. This requires different types handling.

For starters, don't brake on ice. As established earlier, your brakes will be much less effective on ice and attempting apply your brakes will only thrust your momentum forward. Instead, approach icy patches slowly (which you should be doing anyway) and coast along them. On straight stretches of road, you shouldn't need to use the steering wheel much at all to guide your car. In a curve, you can turn the wheel to guide your car, but be aware of skidding (and how to escape various skids). If you begin to skid, turn your wheels into the skid. This allows your wheels to gain a bit more friction and guide the wheels that are skidding:

Friction is the major player here. If a car is moving across snow or ice in any direction other than that which it's pointed, it is skidding (or sliding), and a sideways load is being imposed against the tires. This causes a lot of friction, even if the tires are allowed to turn. By turning the front wheels into the direction of the skid, the front wheels will become aligned with the direction of travel. No more side load will be on the front tires, and they will roll freely again (very little friction). But the back tires will still be sliding sideways, and the greater friction back there will cause the back end to trail directly behind the front end just like a badminton birdie, thus the car straightens out. As the car straightens, the front wheels must be kept aligned with the direction of travel, or a skid in the opposite direction will quickly develop.

If you find yourself in the worst-case scenario—a stopped car on an icy patch with no traction—you can get out, but it's going to be a difficult endeavor. We've previously suggested using your car's floor mats to gain traction, but if you can carry grainy items like cat litter or sand in your trunk, you can spread these around to get your car moving again as well. If you have room, you should also try backing up to an area that does have traction and gaining momentum before tackling an icy patch again. It will be a lot easier than powering through the ice by flooring the gas.

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The most important thing to keep in mind when driving on ice and snow is that your car is going to behave very differently and you need to be able to respond to your car's needs. If you're not comfortable taking a two ton vehicle on an ice skating date, stay at home or find a place to camp out until the ice can melt and emergency services can clear the road. Be sure to check out our previous post with more tips on how to stay safe.